1. Field of Invention
This invention relates to an exposure apparatus, and more particularly to an exposure apparatus in which optical elements, including refracting elements, and gas gaps are arranged along the optical path extending from a mask to a photosensitive substrate, where the portion of the optical path occupied by the air gaps is relatively long.
2. Description of Related Art
In manufacturing of semiconductor wafers using an exposure apparatus, a demand for more minute patterns and further improved image quality has been arising. An example of such high-quality exposure apparatus is disclosed in Japanese Patent Publication (after Examination) No. 7-11512. In this publication, reflecting elements are used as a part of the projection-optical system in order to increase the optical path length without increasing the mechanical dimensions of the projection-optical system, thereby satisfactorily correcting the coma, the field curvature, and the distortion of the system.
Meanwhile, a change of the environmental temperature of the projection-optical system surrounding causes the image quality to deteriorate due to the fluctuation of the index of refraction of the air. To overcome this problem, U.S. pat. No. 4,616,908 discloses a technique for reducing the variation of the index of refraction by filling the environmental space surrounding the projection-optical system (i.e., the microlithographic system) with helium gas.
However, the U.S. Pat. No. 4,616,908 does not disclose the practical and preferable range of the total length of the projection-optical system, or the preferable wavelength of the light beam used in that projection-optical system. For this reason, it is not clear from the description what is the optimum condition of the gas section filled with helium gas for obtaining the optimum resolution. In addition, the invention disclosed in this publication is applied, according to its embodiment, to projection-optical systems that consist of only refracting optical elements. In general, a microlithographic system consisting of only refracting optical elements inherently has less deterioration of image quality due to the air fluctuation.
In contrast, a projection-optical system having reflecting elements, which has a long path length and a high numerical aperture (N.A.), has a large wavefront aberration due to the air fluctuation because the volume occupied by the air gap and the diameter of the light flux become large in the optical path. U.S. Pat. No. 4,616,908 does not teach the practical analysis of the relation between the volume of the air gap in the optical path and the aberrations.